Many of the potential end uses for bi-component fibers require that the fibers be in staple (i.e.--cut) form for processing via traditional yarn spinning (woolen spinning, cotton spinning, etc.) or for processing into non-woven fabrics where the cut fibers are needed. To produce staple fibers via melt spinning economically, it is customary to employ spinnerets having the greatest practical number of holes. A common type of melt spinning spinneret to produce, for instance, two-denier polyester staple fibers (for blending with cotton fibers) would have perhaps 1000 holes in a rectangular area about 7.5 cm wide by 30 cm long. Such a pack would be described as having a high filament density (greater than four holes per square centimeter). Cool air is blown through the fibers below the spinneret across the 7.5 cm dimension.
Bi-component fibers are generally of two different types. The concentric sheath-core type, as the name implies, includes a polymer sheath fiber disposed concentrically about a polymer core fiber. The side-by-side type, on the other hand, includes two polymer fibers disposed side-by-side in parallel relationship. Of course, there are variations of these basic bi-component fiber types, such eccentric sheath-core types wherein the sheath and core fibers are not concentrically disposed. In general, side-by-side fibers are made with two polymers having different shrinkage, retraction or other behavior induced by heat and/or moisture. These fibers are generally referred to as self-crimping, since their "bi-metallic" type of behavior causes them to curl when exposed to heat and/or moisture, resulting in a more bulky fibrous mass. Sheath-core fibers with substantial behave in the same way as the side-by-side type, although their curling forces are not so great.
With all bi-component fiber manufacture via melt spinning there has been a problem delivering a supply of different polymers to each spinning orifice while retaining a high density of filaments per unit area of spinneret face. In making staple fibers, if a substantial drop in spinneret filament density is tolerated, much less fiber production will be achieved per spinning station, greatly increasing the capital cost to obtain a given level of fiber production. More spinning stations will therefore be needed, each having polymer pumps, pump drives, temperature control means, polymer piping, quenching facilities, take-off rolls, and related building space. The most difficult type of conjugate spinning is the concentric sheath-core type when one attempts to achieve a high filament density. One object of this invention is to provide a novel spin pack assembly to achieve high filament density while spinning concentric sheath-core fibers.
One effective prior art pack design for producing sheath-core fibers with a low filament density is disclosed in prior U.S. Pat. No. 2,936,482 (Kilian). In that pack design an upper orifice extrudes the filtered core polymer concentrically into a lead-in-hole of a spinning capillary. The sheath polymer is filtered in a second chamber and fed to flow radially outward from a central location through a common space and over a plateau surrounding the lead-in-hole so as to feed in around the core polymer. The two polymers flow together in laminar flow (plug flow) down through the lead-in-hole and then through a final spinning capillary, at which point the polymer emerges into the air and is cooled to form a bi-component fiber as shown in FIG. 14 of the Kilian patent. The Kilian spin pack provides a relatively short distance for each polymer to flow from the filtering chamber to the final spinning orifice, especially so in the case of the core polymer. However, Kilian's spin pack, because of free flow required in the common space for the centrally admitted sheath polymer, is only capable of relatively low spinneret filament densities. It is another object of the present invention to provide an improvement of the Kilian approach to formation concentric sheath-core bi-component filaments wherein a high spinneret filament density is obtained.
In melt spinning of synthetic fibers it is known that pumping the polymer through a filtering media (sand, screens, porous sintered metal, etc.) just prior to fiber extrusion tends to improve spinning performance. In sheath-core fibers, the core polymer generally provides the fiber strength and the sheath polymer has a lower melting temperature, enabling the fibers to be used in a non-woven fabric which can be bonded by subjecting the fabric to a temperature which will melt (or make "tacky") the sheath polymer without causing significant degradation to the strength of the core polymer. With this type of fiber it is very important that the core polymer pass through the final spinning orifice without a long delay after shearing takes place or else relaxation will offset the benefits of shearing. It is another object of the present invention to form sheath-core bi-component filaments in a manner which minimizes the polymer residence time in the spin pack.
There are two prior art spin packs which achieve high filament density in making sheath-core fibers. In U.S. Pat. No. 3,807,917 (Shimoda et al.), the spin pack assembly is designed for spinning polymer solutions, not melts, and no provision is made to keep a short residence time from the filtering and shearing media to the orifice. In fact, the Shimoda et al. assembly has no provision for filtering at all. In U.S. Pat. No. 4,052,146 (Sternberg), which is designed for molten polymer and does provide fairly high filament density, no means are disclosed for filtering or shearing the polymer and keeping the residence time short from shearing to fiber extrusion.